Karl Fogel at
Karthik Chikmagalur's post "Batteries Included With Emacs" (https://karthinks.com/software/batteries-included-with-emacs/) taught me a bunch of useful things in Emacs that I didn't know about before, and I've been using Emacs for decades.
(For some that might be an indictment of Emacs; I think it's an endorsement!)
Oh, and I just found out he did a followup post: https://karthinks.com/software/more-batteries-included-with-emacs/